Three effects of SO2 on the upper respiratory tract of the chick will be studied: 1. We will develop further tests of the hypothesis that SO2 exposure causes a slowing of mucociliary transport by altering the structure of mucus in such a way that the secretory units are less readily released from the gland cells and become increasingly anchored within the cells. 2. The phenomenon of homeostasis (return of mucociliary transport rates toward normal following continued SO2 exposure) will be investigated in individual chickens to determine whether the consistent variation between individual chickens is related to age and sex of the chickens, whether it is accompanied by histochemical and ultrastructural changes in the mucus, and whether variation in sulfite oxidase activity in the nasal tissues is related to the homeostatic mechanism. 3. We will determine the intensity and duration of SO2 exposure which allows for spread of an established infection which is localized in one turbinate to the other side of the nose and to the trachea. This "breakpoint" may be related to a similar breakpoint in homeostatic adaptation (and to the proposed mechanism of mucociliary transport).